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Making a Density Column

This experiment is one of the classics that is still a lot of fun to play with. It has to do with density. Density can be a difficult concept to grasp, but it is much easier once you have played with a density column.

Imagine that I have three cups that are the same size. If I fill one cup with water, one with sand, and one with lead. Would they all weigh the same? No, of course not. A cup of sand weighs more than a cup of water. Sand is denser than water. Lead is denser than sand, so a cup of lead weighs more than a cup of sand.

Density also explains why things float. A cubic inch of wood weighs less than a cubic inch of water, so the wood will float. The wood is less dense than the water. A cubic inch of lead weighs more than a cubic inch of water, so the lead will sink. Lead is more dense than water.

Now, what if you tried that with different materials, and different liquids?

We will use this idea to make several layers of liquid that will let is compare the density of different objects. You will need:

  • a tall, clear glass or jar
  • water
  • vegetable oil
  • rubbing alcohol
  • corn syrup or other sugary syrup
  • a spoon or ladle
  • a variety of small objects such as: cork, rubber, plastic, bread, corn flakes, ice, a piece of apple, etc.

Pour about one and a half inches of corn syrup into the glass. Place the ladle just at the top of the layer of syrup and gently pour in another inch or two of water. The water should form a layer on top of the corn syrup, and by pouring it into the ladle, you keep from mixing the two layers. Keeping the ladle just at the surface of the liquid, add an inch or two of cooking oil to form the next layer. Once that is settled, add an inch of rubbing alcohol for the top layer. When you finish, place the glass on the table and look at it from the side. You should be able to easily see the different layers of liquid.

The corn syrup is the densest, so it is on the bottom. Next is the water, then the oil, and last is the alcohol, which is the least dense. Now, drop a small piece of bread into the glass. It will float for a second and then as it soaks up the alcohol, it will sink. It does not go all the way to the bottom. Instead, it sinks down to the top of the oil. Soggy bread is denser than the alcohol, but less dense than the oil, so it floats at the boundary of the two.

Drop in raisins, buttons, olives, pieces of plastic, coins, corks, and any other small objects that won't be hurt by putting them into the liquid. Notice which layer each floats on. That tells you their relative density.

Probably the most interesting thing for me was a cornflake, which floated first and then sank into the alcohol as it got wet. It floated on the oil for a while and then as it soaked up some oil, it sank through that. As it reached each layer, it floated until it soaked up some of the new liquid. This increased its density, causing it to sink into the next layer.

What if you added drops of another liquid? Would that work? Try adding a lot of salt to a spoon full of water. Then add a bit of food coloring or fruit juice to color the saltwater. Carefully, add a drop or two to the glass and watch what happens. Try the same thing with a spoon full of water that has a lot of sugar in it.

Before you start pouring everything in the house into your glass, be sure to keep safety in mind.
  • Do not mix different kinds of cleaning solutions, especially if they contain bleach.
  • Be sure that your glass is clearly marked, so no one accidentally picks it up to take a drink. Think what an unpleasant surprise that would be!

Science Fair Thoughts

Are denser foods more nutritions? Does the density of fruit change as it ripens? Does raising or lowering the temperature of the column cause things to float at different levels? If so, what does that tell you?